While writing up an entry for Hactar's scheme from Life The Universe And Everything, I had a moment of fridge brilliance over a plot that'd confused me for years. At first, his backup plan seems to be a Gambit Roulette: how could he have possibly known that Arthur would take the Ashes back several days into the past, and then find the cricket ball, and decide to fulfill his fantasy of bowling at Lord's Ground, and have a war robot waiting there, and be unable to stop himself... I mean, come on! Then while typing, it hit me. Hactar's ability to manipulate people's thoughts depends on how close they are to the dust cloud. He says himself that he needed the leaders of Krikkit to live in the orbital stations to increase his mental hold on them. Arthur and Trillian, though, walked right into the dust cloud. They breathed in his nanotechology as if it were air, it kept them alive in the vacuum of space. So of course, he could manipulate them in ways he never could've done with Krikkit. Why was Arthur so determined to take the Ashes back in time, and to bowl at Lord's Ground with the war robot, and why couldn't he stop running once he realized what was happening? Because Hactar had planted an unconscious command for him to do so. Hactar wasn't just correctly guessing all of Arthur's reactions - he actually made Arthur do those things. — BritBllt
Also from Life, the Universe and Everything, there's a scene where Trillian is on Slartibartfast's ship studying the viewscreen intently while the heroes talk about how to stop Krikkit from destroying the universe. She keeps rapidly flipping between three images with increasing confusion: the planet Krikkit, the star it orbits and the dust cloud. Later, after she reads about Hactar, she's put all the pieces together. But the book never came right out and said why she was flipping between those three images, what was so important about them. It's only when you think about it later that it makes sense. She's an astrophysicist, and what she's looking at is physically impossible. By the time life evolved on Krikkit, the stellar wind from its sun should have long since blown away any dust clouds in the star system. There's no natural way a perfectly spherical, hollow dust cloud could keep surrounding the star system like that for billions of years. That's her first big clue that it's not natural at all, that the dust cloud itself has something to do with what's going on.
Also from Hitchhiker's Guide, it took me two years to get the "ask a glass of water" joke after first seeing it in the TV series. It took a friend of mine 12. —Pumpkinetics
Eight years, here! Thank God it wasn't just me.
I got the "glass of water" bit fairly immediately, but the "military academy" joke took me about four years to sort out. —Wack'd
The film version of Arthur is happy to explore the universe. Why? He wanted to go back home - and did. Leaving Earth this time was his choice.
Little bit of fridge funny from the book. Slatibartfast's ship's teleporters are in the bathroom of the restaurant that powers the ship by doing impossible math in regards the bill. One of the things that can be done with a bill is to do a runner away from it. Usually via the bathroom window.
The method they use for flying (throwing yourself at the ground and missing) is an effective description of how a spacecraft remains in orbit: it is actually falling, but its horizontal velocity is high enough that its curved trajectory never intersects the surface of the Earth. This creates weightlessness inside the spacecraft, which allows its occupants to float and fly within that environment. Also, the trajectory (ascending and descending parabolas) of the Vomit Comet planes that simulate weightlessness, allowing occupants to fly within the enclosed environment of the plane, consists of throwing the plane at the ground (the descending part, where weightlessness is experienced) and missing (the ascending part, where the airplane returns to the previous altitude and a higher gravity is experienced).
Why is Arthur unaffected by the Vogons' poetry? Why, because Earth had the worst poet in the universe, of course!
He's not exactly unaffected, he screams and groans during the reading as much as Ford does, at least in the radio version.
And in the novel as well. It's just that Ford wouldn't have thought to try to bareface his way through Jeltz's interview following the reading.
From the movie: the Vogons are the way they are because they evolved up on a planet where every time you try to think, you get whacked in the face by a shovel. (This has been confirmed via Word of God.)
You may wonder why anyone would go to the effort of inventing Peril-Sensitive Sunglasses, until you realize they are a perfect defense against the Ravenous Bugblatter Beast of Trall.
Arthur, upon seeing Slartibartfast's spaceship, seems to be relatively unaffected by the Somebody Else's Problem field. In fact, he sees right through it. This might just be because, at this point in his life, he immediately assumes that something like a spaceship made to look like an Italian bistro either is his problem, or will rapidly become his problem.
It helps if you know that Adams once wrote for Doctor Who; armed with that knowledge, the Somebody Else's Problem field is an obvious and rather blatant Expy of the show's "perception filter", which can't fool someone who is expecting things to be weird.
The Hitchhiker's trilogy, of all things, has several. For instance, there's one side-story about a scientist who spends his entire career trying to prove his theory that ballpoint pens are actually an alien lifeform, and that when you lose them, it's because they've escaped and are trying to return to their home planet. He claims to have found this planet... but when people check, they find noone but a little old man who claims the ballpoint pen thing wasn't true, but he's lying and gets a large sum of money from Zaphod Beeblebrox's secondhand pen business. So: mass slavery of sentient aliens or genocide? Take your pick!
Or Zaphod is stealing his poor, deluded friend's pens and reselling them at a profit.
I thought it was widely understood that ballpoint pens are the larval form of coat hangers.
In an in-universe subversion, Fenchurch tells a story about how she had a poster of a sea otter pulling a raft full of animals when she was young, and how she would feel sorry for the sea otter for having to pull such a load. Then she realized when she grew up that the raft actually had a sail on it, and the otter wasn't pulling it at all.
Those poor folks on the planet that got caught in a game of star billiards.
The Vogons, by being "professionally unpleasant", effectively run the universe. This is the same species that would blow up a planet — not just so that it and it's many inhabitants don't exist but so that they never existed, in this and all possible universes — rather than mess up the paperwork.
In recording the Tertiary Phase of the Radio Series, Director Dirk Maggs had the brilliant idea of posthumously fulfilling Douglas Adams' wish to voice the character "Agrajag", using edited voice recordings of Adams saying Agrajag's lines when he narrated an audiobook of "Life, The Universe and Everything". Given that Agrajag is a character who is horribly angry and resentful because fate has dictated that his death be the fault of Arthur Dent, no matter what time or place his soul is reincarnated into, one can only imagine the existential horror undergone by Dent's one and only voice actor, Simon Jones. Picture this: acting in response to the pre-recorded voice of one of your best friends, now dead, as he loudly expounds on how you've been unknowingly murdering him over and over again and his fervent desire to kill you...
God's final message to His Creation, located on an alien planet, is written in British English. It's spelled out letter-by-letter, so there's no chance that it's the setting's Translator Microbes. Nobody seems to find this strange.